# Copyright (c) 2017-2019 Uber Technologies, Inc.
# SPDX-License-Identifier: Apache-2.0

import math
from typing import Any, Callable, Optional, Tuple

import torch
from torch.optim.optimizer import Optimizer


class ClippedAdam(Optimizer):
    """
    :param params: iterable of parameters to optimize or dicts defining parameter groups
    :param lr: learning rate (default: 1e-3)
    :param Tuple betas: coefficients used for computing
        running averages of gradient and its square (default: (0.9, 0.999))
    :param eps: term added to the denominator to improve
        numerical stability (default: 1e-8)
    :param weight_decay: weight decay (L2 penalty) (default: 0)
    :param clip_norm: magnitude of norm to which gradients are clipped (default: 10.0)
    :param lrd: rate at which learning rate decays (default: 1.0)
    :param centered_variance: use centered variance (default: False)

    Small modification to the Adam algorithm implemented in torch.optim.Adam
    to include gradient clipping and learning rate decay and an option to use
    the centered variance (see equation 2 in [2]).

    **References**

    [1] `A Method for Stochastic Optimization`, Diederik P. Kingma, Jimmy Ba
        https://arxiv.org/abs/1412.6980

    [2] `A Two-Step Machine Learning Method for Predicting the Formation Energy of Ternary Compounds`,
        Varadarajan Rengaraj, Sebastian Jost, Franz Bethke, Christian Plessl,
        Hossein Mirhosseini, Andrea Walther, Thomas D. Kühne
        https://doi.org/10.3390/computation11050095
    """

    def __init__(
        self,
        params,
        lr: float = 1e-3,
        betas: Tuple = (0.9, 0.999),
        eps: float = 1e-8,
        weight_decay=0,
        clip_norm: float = 10.0,
        lrd: float = 1.0,
        centered_variance: bool = False,
    ):
        defaults = dict(
            lr=lr,
            betas=betas,
            eps=eps,
            weight_decay=weight_decay,
            clip_norm=clip_norm,
            lrd=lrd,
            centered_variance=centered_variance,
        )
        super().__init__(params, defaults)

    def step(self, closure: Optional[Callable] = None) -> Optional[Any]:
        """
        :param closure: An optional closure that reevaluates the model and returns the loss.

        Performs a single optimization step.
        """
        loss = None
        if closure is not None:
            loss = closure()

        for group in self.param_groups:
            group["lr"] *= group["lrd"]

            for p in group["params"]:
                if p.grad is None:
                    continue
                grad = p.grad.data
                grad.clamp_(-group["clip_norm"], group["clip_norm"])
                state = self.state[p]

                # State initialization
                if len(state) == 0:
                    state["step"] = 0
                    # Exponential moving average of gradient values
                    state["exp_avg"] = torch.zeros_like(grad)
                    # Exponential moving average of squared gradient values
                    state["exp_avg_sq"] = torch.zeros_like(grad)

                exp_avg, exp_avg_sq = state["exp_avg"], state["exp_avg_sq"]
                beta1, beta2 = group["betas"]

                state["step"] += 1

                if group["weight_decay"] != 0:
                    grad = grad.add(p.data, alpha=group["weight_decay"])

                # Decay the first and second moment running average coefficient
                exp_avg.mul_(beta1).add_(grad, alpha=1 - beta1)
                grad_var = (grad - exp_avg) if group["centered_variance"] else grad
                exp_avg_sq.mul_(beta2).addcmul_(grad_var, grad_var, value=1 - beta2)

                denom = exp_avg_sq.sqrt().add_(group["eps"])

                bias_correction1 = 1 - beta1 ** state["step"]
                bias_correction2 = 1 - beta2 ** state["step"]
                step_size = group["lr"] * math.sqrt(bias_correction2) / bias_correction1

                p.data.addcdiv_(exp_avg, denom, value=-step_size)

        return loss
